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Ultimate solar system could contain 60 Earths

By Jacob Aron

Why settle for one habitable planet, when you can have 60? An astrophysicist has designed the ultimate star system by cramming in as many Earth-like worlds as possible without breaking the laws of physics. Such a monster cosmic neighbourhood is unlikely to exist in reality, but it could inspire future exoplanet studies.

Sean Raymond of Bordeaux Observatory in France started his game of fantasy star system with a couple of ground rules. First, the arrangement of planets must be scientifically plausible. Second, they must be gravitationally stable over billions of years&colon; there is no point in putting planets into orbit only to watch them spiral into the sun.

“The arguments were based on the recent scientific literature as well as some simple calculations I did,” says Raymond. In some cases it was impossible to choose between two scenarios because of a lack of data, so he just picked the one he liked best.

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To start with he chose a red dwarf star as the system’s host because they have a lower mass than stars like our sun and so live longer, giving a stable habitable zone – the region around a star in which liquid water can exist.

Next, he used a couple of tricks to boost the planetary potential of his system. An Earth-sized planet can also have an almost Earth-sized moon, with the two worlds orbiting around a central point. What’s more, two pairs of planets can orbit a star at the same distance, provided that they are separated by 60 degrees, thanks to a couple of gravitationally stable points. In our solar system these points are normally inhabited by asteroids, rather than planets, but nothing rules out a multiple planet scenario. Objects in this configuration are known as Trojans – Jupiter has thousands, and even Earth has one.

There is room for six of these orbital configurations in the habitable zone of a red dwarf, giving a total of 24 habitable planets in one system. But it turns out there is also another way to build a packed system&colon; Jupiters.

A star system with four gas giants could support 36 habitable worlds (Image&colon; planetplanet.net)

Gas giants such as Jupiter are not habitable to life as we know it, but they can be orbited by Earth-like moons. In our solar system, Europa and Enceladus, which orbit Jupiter and Saturn, respectively, are prime candidates for extraterrestrial life. Raymond calculates that a red dwarf could hold four Jupiter-like planets, each with five Earth-like moons. What’s more, the Trojan trick can allow another two Earth-like planets on either side of the orbiting Jupiters, upping the total number of habitable worlds around the red dwarf to 36.

Finally, Raymond turned his star system into a binary one, with two red dwarfs separated by roughly the distance from our sun to the edge of the solar system. Theory allows one star to carry the Earth-only configuration, and the other to carry the Earth-plus-Jupiters configuration. This creates the ultimate star system, with 60 habitable planets to choose from.

“It is thought provoking,” says Mikko Tuomi of the University of Hertfordshire in Hatfield, UK, who helped to discover the star system with the largest number of known planets, but the odds of something like it actually forming in the real universe are slim to none. “This would be due to the lack of matter at or near the habitable zone in the accretion disk from which planets form,” says Tuomi. Sufficiently advanced aliens could build a system like this, he says, but it is not clear why they would bother.

“I admit that it would be extremely fortuitous for nature to produce a system that was so spectacular,” says Raymond. “Still, each piece of the system is plausible and even expected from simulations of planetary formation.”

Coming up with the system has also thrown up new scientific questions, he says. “I ended up doing a lot of research into the different pieces of the puzzle, and coming at it from this point of view gave me some new ideas I’m planning to test in the future.”